The present invention relates to indirect printing systems and more particularly to compositions suitable for the treatment of intermediate transfer members.
Digital printing techniques have been developed that allow a printer to receive instructions directly from a computer without the need to prepare printing plates, as in the more traditional offset methods. Various printing systems exist which may use either dry inks, such as the toners used in laser printers, or liquid inks having either organic or aqueous solvents or carriers. Such technologies may rely on direct application of inks in an image pattern onto paper or any other substrate, as in ink jetting commonly used in home and office printers, or they may rely on indirect printing in which a mirror image is first formed on an intermediate member and then transferred therefrom to the substrate. Such indirect method, more frequent in commercial settings, is exemplified by the liquid electro-photographic process in which an electrostatic image is first produced on an electrically charged image bearing cylinder by exposure to laser light. The electrostatic charge attracts oil-based inks to form a color ink image which is then transferred by way of a blanket cylinder onto the printing substrate (e.g., paper, cardboard, plastic etc.).
Such processes suffer from drawbacks. In liquid ink processes, for instance, the use of organic-based solvents creates a challenging safety and environmental concern. Direct ink jetting of liquid inks, typically aqueous, yields, on the other hand, limited resolution due to wicking of the inks into fibrous substrates, such as paper. Though such problems might be partially addressed by the use of substrates with special coatings engineered to absorb the liquid ink in a controlled fashion or to prevent its penetration below the surface of the substrate, such a solution is not suitable for certain printing applications and its cost makes it not viable for commercial printing. Furthermore, the use of coated substrates creates its own problems in that the surface of the substrate remains wet and additional costly and time consuming steps are needed to dry the ink, so that it is not later smeared as the substrate is being handled, for example stacked or wound into a roll. In addition, excessive wetting of the substrate causes cockling and makes printing on both sides of the substrate (also termed perfecting or duplex printing) difficult, if not impossible.
Moreover, inkjet printing directly onto porous paper, or other fibrous material, results in poor image quality because of variation of the distance between the print head and the surface of the substrate.
Using an indirect printing technique overcomes many problems associated with inkjet printing directly onto the substrate. It allows the distance between the surface of the intermediate image transfer member and the inkjet print head to be maintained constant and reduces wetting of the substrate, as the ink can be dried on the surface of the intermediate transfer member (also termed the release layer) before being applied to the substrate.
The present Applicant has recently disclosed printing processes wherein inks including an organic polymeric resin and a coloring agent in an aqueous carrier are jetted at an image forming station onto an intermediate transfer member having a hydrophobic release layer. The ink image so formed is dried to leave a residue film of resin and coloring agent before being transferred to the desired substrate at an impression station. Such processes were concerned with balancing factors having contradictory requisites to achieve print quality. For instance, the ink droplets need to sufficiently adhere to the release layer at the image forming station not to be affected by the movement of the transfer member, whereas the dried ink films need to easily detach therefrom at the impression station. While silicone coated transfer members are preferred to facilitate transfer of the dried image to the final substrate, their hydrophobicity causes aqueous ink droplets to bead on the transfer member. This makes it more difficult to remove the water in the ink and also results in a small contact area between the droplet and the blanket that renders the ink image unstable during rapid movement. The earlier disclosed printing processes of the Applicant are believed to be based on spontaneous and reversible electrostatic mechanisms. The proper selection of the chemical compositions of the ink and of the surface of the intermediate transfer member results in attractive intermolecular forces between molecules on the outer surface of each droplet and on the surface of the intermediate transfer member. It was observed that such interactions, though substantially devoid of irreversible chemical reactions, are sufficient to counteract the tendency of the flattened disk-shaped ink film produced by each impinging droplet to bead under the action of the surface tension of the aqueous carrier, without causing each droplet to spread by wetting the surface of the intermediate transfer member. This subtle equilibrium can be optionally achieved with the assistance of a chemical agent being applied to the surface of the release layer before jetting of the ink droplet, the composition comprising such agent being referred to as a conditioning fluid.
Advantageously, the final image quality on the substrate obtained in printing systems based on the previously described processes is less affected by the physical properties of the substrate and benefit from various other advantages as a result of the image remaining above the substrate surface.
The present invention is concerned with chemical agents that can effectively treat the release layer of intermediate transfer members in indirect printing systems.